1. Field of the Invention
The present invention relates generally to a lean burn control system for an internal combustion engine, and more specifically, to a lean burn control system for an internal combustion engine, wherein surging in the engine detected in a lean burn feedback control is eliminated by correcting a target air-fuel ratio of an air-fuel mixture gas supplied to the engine.
2. Description of the Prior Art
A lean burn control system for an internal combustion engine is known, which controls an air-fuel ratio of the air-fuel mixture gas to a lean side with respect to the stoichiometric air-fuel ratio for improving exhaust emission and fuel consumption. In general, when the air-fuel ratio is controlled toward the lean side beyond a lean limit defined by an engine characteristic, combustion in the engine becomes unstable to cause surging. This lean limit is lowered, i.e. shifted toward a richer side, due to deterioration of the engine.
In view of this, technique has been proposed in such as U.S. Pat. No. 4,653,451, wherein the lowering of the engine lean limit is determined when the surging is detected based on outputs of a lean mixture sensor during the lean burn feedback control of the air-fuel ratio. When it is so determined, a target air-fuel ratio of the air-fuel mixture gas is decreased to a richer side.
It is to be appreciated that the surging is likely to be induced not only when the engine lean limit is lowered, but also when an output characteristic of the lean mixture sensor is changed. This change in the lean mixture sensor output characteristic is caused due to deterioration of the lean mixture sensor itself and environmental variation, such as, variation in the atmospheric pressure therearound. For example, an output level of the lean mixture sensor lowers due to the deterioration thereof or at the highlands so that the air-fuel ratio is detected to be richer than an actual value. Accordingly, the system judges the air-fuel ratio to be still on a richer side than a target air-fuel ratio even when the target air-fuel ratio has been actually reached, so as to control the air-fuel ratio more to the lean side. As a result, the air-fuel ratio is controlled to be excessively lean, leading to generation of the surging.
In the conventional system disclosed such as in the foregoing U.S. Patent, however, the surging is dealt with by decreasing the target air-fuel ratio without judging whether it is caused by the lowering of the engine lean limit or the output characteristic variation of the lean mixture sensor. As a result, since the target air-fuel ratio is corrected to be decreased even when the surging is caused by the output characteristic variation of the lean mixture sensor, when the system is shifted to an open loop control, the air-fuel ratio is controlled to be unnecessarily richer due to the continued decreasing correction of the target air-fuel ratio, leading to increment of NOx emission.